This invention relates to the fields of phone line termination circuits and isolation systems for use in selectively isolating electrical circuits from one another. More particularly, this invention relates to isolation systems having capacitor-coupled isolation barriers for phone line termination circuits. This invention is useful in, for example, telephony, medical electronics and industrial process control applications.
Electrical isolation barriers can be identified in many industrial, medical and communication applications where it is necessary to electrically isolate one section of electronic circuitry from another electronic section. In this context isolation exists between two sections of electronic circuitry if a large magnitude voltage source, typically on the order of one thousand volts or more, connected between any two circuit nodes separated by the barrier causes less than a minimal amount of current flow, typically on the order of ten milliamperes or less, through the voltage source. An electrical isolation barrier must exist, for example, in communication circuitry which connects directly to the standard two-wire public switched telephone network and that is powered through a standard residential wall outlet. Specifically, in order to achieve regulatory compliance with Federal Communications Commission Part 68, which governs electrical connections to the telephone network in order to prevent network harm, an isolation barrier capable of withstanding 1000 volts rms at 60 Hz with no more than 10 milliamps current flow, must exist between circuitry directly connected to the two wire telephone network and circuitry directly connected to the residential wall outlet.
In many applications there exists an analog or continuous time varying signal on one side of the isolation barrier, and the information contained in that signal must be communicated across the isolation barrier. For example, common telephone network modulator/demodulator, or modem, circuitry powered by a residential wall outlet must typically transfer an analog signal with bandwidth of approximately 4 kilohertz across an isolation barrier for transmission over the two-wire, public switched telephone network. The isolation method and associated circuitry must provide this communication reliably and inexpensively. In this context, the transfer of information across the isolation barrier is considered reliable only if all of the following conditions apply: the isolating elements themselves do not significantly distort the signal information, the communication is substantially insensitive to or undisturbed by voltage signals and impedances that exist between the isolated circuitry sections and, finally, the communication is substantially insensitive to or undisturbed by noise sources in physical proximity to the isolating elements.
High voltage isolation barriers are commonly implemented by using magnetic fields, electric fields, or light. The corresponding signal communication elements are transformers, capacitors and opto-isolators. Transformers can provide high voltage isolation between primary and secondary windings, and also provide a high degree of rejection of lower voltage signals that exist across the barrier, since these signals appear as common mode in transformer isolated circuit applications. For these reasons, transformers have been commonly used to interface modem circuitry to the standard, two-wire telephone network. In modem circuitry, the signal transferred across the barrier is typically analog in nature, and signal communication across the barrier is supported in both directions by a single transformer. However, analog signal communication through a transformer is subject to low frequency bandwidth limitations, as well as distortion caused by core nonlinearities. Further disadvantages of transformers are their size, weight and cost.
The distortion performance of transformer coupling can be improved while reducing the size and weight concerns by using smaller pulse transformers to transfer a digitally encoded version of the analog information signal across the isolation barrier, as disclosed in U.S. Pat. No. 5,369,666, xe2x80x9cMODEM WITH DIGITAL ISOLATIONxe2x80x9d (incorporated herein by reference). However, two separate pulse transformers are disclosed for bidirectional communication with this technique, resulting in a cost disadvantage. Another disadvantage of transformer coupling is that additional isolation elements, such as relays and opto-isolators, are typically required to transfer control signal information, such as phone line hookswitch control and ring detect, across the isolation barrier, further increasing the cost and size of transformer-based isolation solutions.
Because of their lower cost, high voltage capacitors have also been commonly used for signal transfer in isolation system circuitry. Typically, the baseband or low frequency analog signal to be communicated across the isolation barrier is modulated to a higher frequency, where the capacitive isolation elements are more conductive. The receiving circuitry on the other side of the barrier demodulates the signal to recover the lower bandwidth signal of interest. For example, U.S. Pat. No. 5,500,895, xe2x80x9cTELEPHONE ISOLATION DEVICExe2x80x9d (incorporated herein by reference) discloses a switching modulation scheme applied directly to the analog information signal for transmission across a capacitive isolation barrier. Similar switching circuitry on the receiving end of the barrier demodulates the signal to recover the analog information. The disadvantage of this technique is that the analog communication, although differential, is not robust. Mismatches in the differential components allow noise signals, which can capacitively couple into the isolation barrier, to easily corrupt both the amplitude and timing (or phase) of the analog modulated signal, resulting in unreliable communication across the barrier. Even with perfectly matched components, noise signals can couple preferentially into one side of the differential communication channel. This scheme also requires separate isolation components for control signals, such as hookswitch control and ring detect, which increase the cost and complexity of the solution.
The amplitude corruption concern can be eliminated by other modulation schemes, such as U.S. Pat. No. 4,292,595, xe2x80x9cCAPACITANCE COUPLED ISOLATION AMPLIFIER AND METHOD,xe2x80x9d which discloses a pulse width modulation scheme; U.S. Pat. No. 4,835,486 xe2x80x9cISOLATION AMPLIFIER WITH PRECISE TIMING OF SIGNALS COUPLED ACROSS ISOLATION BARRIER,xe2x80x9d which discloses a voltage-to-frequency modulation scheme; and U.S. Pat. No. 4,843,339 xe2x80x9cISOLATION AMPLIFIER INCLUDING PRECISION VOLTAGE-TO-DUTY CYCLE CONVERTER AND LOW RIPPLE, HIGH BANDWIDTH CHARGE BALANCE DEMODULATOR,xe2x80x9d which discloses a voltage-to-duty cycle modulation scheme. (All of the above-referenced patents are incorporated herein by reference.) In these modulation schemes, the amplitude of the modulated signal carries no information and corruption of its value by noise does not interfere with accurate reception. Instead, the signal information to be communicated across the isolation barrier is encoded into voltage transitions that occur at precise moments in time. Because of this required timing precision, these modulation schemes remain analog in nature. Furthermore, since capacitively coupled noise can cause timing (or phase) errors of voltage transitions in addition to amplitude errors, these modulation schemes remain sensitive to noise interference at the isolation barrier.
Another method for communicating an analog information signal across an isolation barrier is described in the Silicon Systems, Inc. data sheet for product number SS173D2950. (See related U.S. Pat. No. 5,500,894 for xe2x80x9cTELEPHONE LINE INTERFACE WITH AC AND DC TRANSCONDUCTANCE LOOPSxe2x80x9d and 5,602,912 for xe2x80x9cTELEPHONE HYBRID CIRCUITxe2x80x9d, both of which are incorporated herein by reference.) In this modem chipset, an analog signal with information to be communicated across an isolation barrier is converted to a digital format, with the amplitude of the digital signal restricted to standard digital logic levels. The digital signal is transmitted across the barrier by means of two, separate high voltage isolation capacitors. One capacitor is used to transfer the digital signal logic levels, while a separate capacitor is used to transmit a clock or timing synchronization signal across the barrier. The clock signal is used on the receiving side of the barrier as a timebase for analog signal recovery, and therefore requires a timing precision similar to that required by the analog modulation schemes. Consequently one disadvantage of this approach is that noise capacitively coupled at the isolation barrier can cause clock signal timing errors known as jitter, which corrupts the recovered analog signal and results in unreliable communication across the isolation barrier. Reliable signal communication is further compromised by the sensitivity of the single ended signal transfer to voltages that exist between the isolated circuit sections. Further disadvantages of the method described in this data sheet are the extra costs and board space associated with other required isolating elements, including a separate high voltage isolation capacitor for the clock signal, another separate isolation capacitor for bidirectional communication, and opto=isolators and relays for communicating control information across the isolation barrier.
Opto-isolators are also commonly used for transferring information across a high voltage isolation barrier. Signal information is typically quantized to two levels, corresponding to an xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d state for the light emitting diode (LED) inside the opto-isolator. U.S. Pat. No. 5,287,107 xe2x80x9cOPTICAL ISOLATION AMPLIFIER WITH SIGMA-DELTA MODULATIONxe2x80x9d (incorporated herein by reference) discloses a delta-sigma modulation scheme for two-level quantization of a baseband or low frequency signal, and subsequent communication across an isolation barrier through opto-isolators. Decoder and analog filtering circuits recover the baseband signal on the receiving side of the isolation barrier. As described, the modulation scheme encodes the signal information into on/off transitions of the LED at precise moments in time, thereby becoming susceptible to the same jitter (transition timing) sensitivity as the capacitive isolation amplifier modulation schemes.
Another example of signal transmission across an optical isolation barrier is disclosed in U.S. Pat. No. 4,901,275 xe2x80x9cANALOG DATA ACQUISITION APPARATUS AND METHOD PROVIDED WITH ELECTRO-OPTICAL ISOLATIONxe2x80x9d (incorporated herein by reference). In this disclosure, an analog-to-digital converter, or ADC, is used to convert several, multiplexed analog channels into digital format for transmission to a digital system. Opto-isolators are used to isolate the ADC from electrical noise generated in the digital system. Serial data transmission across the isolation barrier is synchronized by a clock signal that is passed through a separate opto-isolator. The ADC timebase or clock, however, is either generated on the analog side of the barrier or triggered by a software event on the digital side of the barrier. In either case, no mechanism is provided for jitter insensitive communication of the ADC clock, which is required for reliable signal reconstruction, across the isolation barrier. Some further disadvantages of optical isolation are that opto-isolators are typically more expensive than high voltage isolation capacitors, and they are unidirectional in nature, thereby requiring a plurality of opto-isolators to implement bidirectional communication.
Thus, there exists an unmet need for a reliable, accurate and inexpensive apparatus for effecting bidirectional communication of both analog signal information and control information across a high voltage isolation barrier, while avoiding the shortcomings of the prior art.
A communication system having a framing pattern to frame data to be transmitted to or from a phone line is provided. The data may be framed on one side of an isolation barrier and a clock signal may be extracted from the framed data stream on the other side of the barrier. The data to be framed is provided from an output of a delta-sigma modulator and the framing pattern utilized is a pattern that is unlikely to match the data stream output of the modulator. Thus, an erroneous detection of the framing pattern is unlikely to occur. The framing pattern is chosen to correspond to the expected modulator output for a full scale input signal that is at a frequency higher than the maximum actual frequency of the input data provided to the modulator.
In one embodiment, a method of providing a communication system that may be coupled to a phone line is provided. This method may include coupling a capacitive isolation barrier between powered circuitry and phone line side circuitry and generating a serial data bit stream within at least one of the powered circuitry or the phone line side circuitry. Further, the method may include framing the serial data bit stream with a plurality of framing bits and transmitting a serial digital bit stream across the isolation barrier, the serial digital bit stream comprising data bits and framing bits.
In another embodiment, a method of framing data in a communication system is provided. This method may include providing first circuitry that may be located on a first side of an isolation barrier, the first circuitry being either phone line side circuitry that may be coupled to phone lines or powered side circuitry that may be coupled to a power source. The method also includes providing a digital data stream within the first circuitry, the digital data stream to be provided from the first circuitry for transmission across the isolation barrier, and framing the digital data stream with a framing pattern, the framing pattern being unlikely to match a bit pattern within the digital data stream.
In yet another embodiment, a method of framing the output data of a delta-sigma modulator is provided. This method may include providing input data to the delta-sigma modulator, the input data having an expected maximum frequency, and framing the output data of the delta-sigma modulator with a framing pattern unlikely to match a bit pattern within the output data.